The harsh and extreme operating conditions of the steelmaking furnaces, i.e. the high temperatures and the inherent problems of handling molten metals as well as the huge production rates, make it difficult, to devise efficient methods and equipment for charging the furnaces, not only with the metallic iron charges in solid or liquid form, but also with diverse additives necessary to refine the molten iron to obtain the desired composition and the characteristics of steel.
Although the invention will be herein described as applied to feeding DRI to Electric Arc Furnaces (EAF), it will be understood that the invention can be easily modified and adapted to other types of furnaces and materials.
Many methods of feeding EAF's have been proposed and tried over the years. In the early days of electric arc steel-making, such furnaces were charged essentially with scrap by means of huge buckets. The furnace was opened by swinging aside its roof and electrodes, and an overhead crane positioned the buckets over the furnace and dumped the scrap in the furnace by opening the bottom of the buckets.
Due to the bulky volume of scrap, when the first charge was melted down, the furnace had to be opened again and a second charge added to the furnace. When this second charge was melted, then even a third opening and charging of the furnace was necessary.
This method had numerous drawbacks; specifically, substantial amounts of energy were lost at each opening due to radiation from the roof, electrodes and metal bath; and productivity of the furnace was severely restricted because of the maneuvering time needed for opening and charging the furnace for each bucket of scrap or prereduced materials.
In more recent years, a number of improvements have been made to the EAF steelmaking practice and consequently productivity has been considerably raised. These improvements include the use of Ultra High Power input to the furnaces which permits optimization of the overall melting operation of the furnace and leaves the refining and secondary metallurgical operations to be done in a separate ladle furnace.
Productivity of EAF's was further raised by continuous charging of scrap or prereduced materials. The iron-bearing material is introduced continuously into the furnace while the first charge is melting, thus obviating the crane maneuvering time.
Many methods for continuously charging EAF's have been proposed. These can be grouped under three general categories:
One method is to feed the EAF by dropping the iron particles through a hole in the roof of the EAF. An example of this method is described in U.S. Pat. No. 3,472,650 to J. G. Sibakin et al. Sibakin teaches a method of continuous charging of discrete iron-bearing material to an electric arc furnace, where the iron-bearing particles are dropped by means of suitable chutes through holes the roof of the furnace. One of the objects of this invention is to stabilize the electric arc by matching the rate of charge with the level of power fed to the furnace.
U.S. Pat. No. 2,805,930 to M. J. Udy is similar to Sibakin but its main object is to shield the furnace walls with the iron-bearing particles to be melted. For this purpose the iron-bearing particles are dropped from openings in the roof of the furnace close to the walls thereof.
This method of feeding EAF's has serious drawbacks. For example, the yield of molten iron produced from the charge is affected by the fact that the iron-bearing particles are entrained by the gases evolving in the furnace and are carried away. This loss of iron may amount up to about 1%. This entrainment has become even more significant due to the widespread use of pollution control equipment for treating the EAF's fumes, because all these gases are drawn into the treating equipment and consequently the gaseous currents inside the EAF are stronger. Also, it is difficult for the fine particles to penetrate the slag layer to reach the molten bath, which being light tend to float on the slag surface.
It has also been proposed to feed iron-bearing materials through the wall of the EAF principally for the purpose of charging the material into the proximity of the arcs, while avoiding the problems involved in feeding the material through the roof in the vicinity of the electrodes.
Examples of this type of feed for EAF's are disclosed in U.S. Pat. No. 3,462,538 to R. E. Pellegrini et al., U.S. Pat. No. 3,681,049 to J. Celada and U.S. Pat. No. 2,657,990 to C. R. Kuzell; as well as Canadian Patent No. 847,454 to G. A. Roeder et al.
Pellegrini describes a method and apparatus for the continuous charging of sponge iron granules into an EAF, wherein the sponge iron particles are injected by imparting to them sufficient energy so as to project them the furnace to the vicinity of the electrodes.
Celada discloses a similar system for injecting sponge iron particles into the furnace by means of a centrifugal slinger comprising a drive belt and a pulley. The sponge iron particles are injected into the vicinity of the arcs. Celada also describes in FIG. 4 another means to carry out his invention, namely a charging tube through which the particles are accelerated by a pressurized fluid, for example, gas, air or steam.
Kuzell describes a feeding system similar to that described by Celada.
Roeder also describes a feeding system where the iron-bearing particles are injected into the furnace by means of an air blast through an aperture in the furnace wall.
It has also been proposed to feed EAF's through the walls and close the surface of the liquid metal or slag usually covering the liquid metal. An example of such a method is described in U.S. Pat. No. 1,338,881 to G. J. Stock. Stock teaches feeding an electric furnace by means of a feed channel through which the iron charge is pushed into the furnace by a piston driven by a crank wheel. The crank wheel is rotated by an electric motor and the speed of the motor is regulated in order to balance the input of electrical energy and the amount of material to be melted with the general purpose of maintaining a constant temperature in the furnace.
A system similar to Stock's but related to calcium carbide production is described in U.S. Pat. No. 1,422,135 to W. B. Rogatz. Rogatz discloses a method of feeding materials into an electric furnace preferably horizontally through an opening in the wall of the furnace and above the molten material in the furnace by means of a piston which injects the material forcefully into the furnace to spread the material broadly across the surface of the molten material. The piston is made of carbon and operates similarly to the device shown by Stock.
U.S. Pat. No. 4,225,745 to Harwell also describes a method for charging small particles of iron and steel directly into molten metal in an arc furnace. Harwell teaches to charge iron oxides, such as mill scale, through a side-delivery tube directly into the bath. The bath is formed in the furnace by suitable charges of materials through the top of the furnace in a conventional manner.
The furnace has a feeder chute entering the furnace well below the normal full metal level. A feeding chamber is provided to introduce large masses of small particles of iron or steel into the feeder chute. A piston pushes the particles into the melting furnace. It will be appreciated that Harwell's charging system is very similar to Stock's, differing in the level of entry into the furnace.
Although at column 4, lines 22 to 29, Harwell mentions that a screw feed is possible, he teaches away from such use because of possible freezup and further teaches away from using even his own feeding system with carbon-containing pellets (because of the high temperatures that may develop in the feeder). On the contrary, the present invention is well suited to feed pellets with any carbon content because such pellets are not fed directly into the bath of molten metal but above the bath.
The problem of freeze-ups mentioned by Harwell makes a direct feed screw not suitable because molten metal may solidify in the feeding chute. While in the present invention applicants have uniquely provided that only slag may contact the feeder tip. Since they realized that solid slag is not as strong as iron or steel, therefore a direct feed screw type conveyor can be used in a manner that provides many advantages and makes the instant invention quite useful.
The other category of feeding systems comprises those actually using a screw type feeder. Examples of these are shown in U.S. Pat. No.: 1,421,185 to R. A. Driscoll; U.S. Pat. Nos. 1,871,848 and 1,902,638 both to E. G. T. Gustafsson.
Driscoll shows a screw type feeder in an electric furnace located above the molten metal bath to feed a powdered fuel. The iron-bearing material however is separately dropped from the top of the roof of the furnace. No details are shown of the cooperation of the fuel feeder and the furnace but from the drawing it can be inferred that the furnace is fixed and that the feeder is fixedly attached to the furnace wall.
Gustafsson discloses a method of melting metals in electric furnaces wherein the metallic material is supplied to the furnace continuously during the course of the heat. Gustafsson also teaches feeding sponge iron into the furnace, and he recognizes the problem of sponge iron floating in the slag layer. The measure he proposes to overcome this problem however is to increase the density of the material and not to apply a positive force to the material in the direction of the molten bath by means of the feeder.
It will be appreciated that although Gustafsson discloses the use of screw type feeders, these are not directed into the molten bath and are not adapted to apply any downward force to the material to supplement the force of gravity. Operation of Gustafsson's feeders is equivalent to dropping it from above. These screws only allow the material to flow out of the hoppers but do not positively push it in a determined direction either directly into the molten bath and/or through the slag layer.
A screw type feeder is also shown in U.S. Pat. No. 4,872,907 to L. D. Areaux who teaches a method and apparatus for charging metal chips into a molten bath, comprising a compacting extruder and a delivery conduit which is resistant to the mass of molten metal and which dips into the molten metal bath. Although Areaux's feeding system is described generally as applied to any metal, he particularizes at column 13, lines 23 to 30, that the invention is to be used in connection with nonmagnetic metal scrap, such as brass, aluminum, aluminum alloys and the like. It will be appreciated that it would be difficult to construct a feeder of this type to be immersed into molten steel. That is why Areaux applies his system to the above-mentioned metals excluding iron and steel.
It will be appreciated that the prior art fails to disclose a feeding system of DRI for a metallurgical furnace which provides the advantages of the invention herein described.